1. Field of the Invention
The present invention relates to printing mechanisms. More specifically, the present invention relates to apparatus and methods for diagnosing printer performance faults.
2. Description of the Related Art
Modern printers have evolved as fast and sophisticated machines that produce high quality text and images from a digital printer file. While modern printers are currently very reliable, that the fact they are complicated machines implies that consumption, wear, component failure, misuse, and environmental factors will cause degradation in performance, partial failure, and even complete failure from time to time over the life cycle thereof. When degradation, partial failure, or complete failure occurs, it is useful to follow a logical troubleshooting procedure, using tools and aids such as those described herein, to accomplish repairs in a timely and economical manner. Since computer printers are complicated machines subject to a large number of failure modes, each repair effort must begin with a diagnostic task to identify the failure mode and isolate the faulty component or components. Accordingly, efficient diagnosis of faults is necessary to reduce the amount of time needed to effect the repair and to add certainty to the determination of the faulty components.
The diagnostic process for a printer is specific to the printing technology employed, as well as the model of the machine being serviced. There are, however, certain procedural similarities in the diagnostic process, whether the technology is a monochromatic or color printer of the inkjet, laser, offset, or any other printer variety. One similarity is the need for an analysis of the printing process at various points in time as the process proceeds in order to identify contributing faults and the source of same. By way of example, consider the modern color laser printer. As is well known in the art, systems and methods for reducing the time needed to isolate faulty components with greater certainty and effect a repair facilitate control and minimization of repair costs.
Laser printing is accomplished by encoding an image as a sequence of light pulses generated by a semiconductor laser. The pulsed beam of light is directed onto a photoconductive drum, belt, or sheet, as a sequence of scanned lines, the spacing of which represents one dimension of the printer resolution; 600 scan lines per inch, for example. The other dimension of the resolution is dependent upon the modulation rate of the light beam, for example, 600 light pulses per inch of scan. The scanning process typically utilizes a rotating mirror, which reflects the pulsed beam of light onto the photoconductive drum, belt or sheet, in a rasterized sequence. Consider the example of a drum type photoconductor for which a photoconductive drum is electrostatically charged prior to the laser beam scan. Areas of the photoconductive drum that are exposed to light are enabled to conduct electricity and the local electrostatic charge is thereby drained to a grounded substrate in the photoconductive drum. The areas of the photoconductive drum that are not illuminated by the pulsed laser beam retain their electrostatic charge. In the case of a color laser printer, three or four rasterized images are typically written to the drum, one for each of the primary colors, yellow, magenta, and cyan, and often an additional one for the color black. Some printing systems may use six or eight primary colors. Individual photoconductive drums for each color can be utilized or a single drum can be utilized multiple times in sequence. The circumference of some photoconductive drums are a fraction of a page, and being seamless, it is common practice for the same area of the drum""s surface to be reused as many as eight or more times per page. In the case of larger photoconductors, depending on the relative size of the drum and print media, the same area of the photoconductor may only be used every Nth (every other, every third, every fourth, etc.) page.
Once the photoconductive drum is exposed with the image by the aforementioned laser beam scan, it is subsequently xe2x80x9cdevelopedxe2x80x9d by transferring toner to the areas with the desired electrostatically charged or discharged condition. Toner is supplied from a toner cartridge in which a reserve of toner is stored. Some of the toner in this reserve is mechanically or otherwise agitated and handled by mechanical, magnetic, electrostatic, and other forces. For transfer from the toner cartridge to the photoconductive drum to occur, a cloud of toner is produced in the vicinity of the photoconductive drum along a lateral region tangential to the drum""s surface. The cloud of toner has a bias voltage applied so as to cause the toner particles to be electrostatically attracted to and repelled from the various charged and discharged areas of the photoconductive drum. In the case of common monocomponent toners, the particles typically have a ferrite core, or, in the case of dual component development systems, the toner is most commonly mixed with ferrite xe2x80x9ccarrierxe2x80x9d particles, that allow the toner particles to be manipulated by both magnetic and electric fields.
While there are a number of ways to generate a cloud of toner, one common approach is to coat the exterior surface of a hollow cylindrical developer sleeve, possessing a suitably thin wall thickness and appropriate surface characteristics, with a thin layer of monocomponent toner particles, or a toner/carrier particle mixture, where the particles are held to the surface of the sleeve by the magnetic field of one or more multi-pole magnets carefully arranged within the cylinder. As the motor driven cylinder rotates about the fixed position interior magnets, the particles on the surface move in response to the varying magnetic field or fields. Sometimes a pair of magnetic poles with the same polarity are arranged a short distance apart within the sleeve. These magnetic poles are each aligned parallel to the axis of the cylinder and run almost the full length of the cylinder. As the rotating sleeve quickly moves, the particles on its surface between the small gap between the magnetic fields causes the various particles to vigorously flip end to end due to their interaction with the magnetic fields, thus producing a cloud of toner particles over the xe2x80x9cgapxe2x80x9d region between the two like magnetic poles. Usually the entire developing mechanism is electrically isolated from its surroundings and a voltage bias signal is applied so that the toner particles within the cloud are electrically charged to a controlled range of values. The resulting charge on the various particles and surfaces is a function of the bias signal applied, the motion of the various particles, the magnetic field, and the properties of the various materials in the developer sleeve, the toner, carrier particles and possibly other factors. The axis of the aforementioned photoconductive drum is also arranged parallel to the developer sleeve at such a distance and angle that the gap region of the developer sleeve and the surface of the photoconductive drum with the electrostatic image are in close proximity to, though usually not contacting, one another. Both the photoconductive drum and the developer sleeve are mechanically rotated, usually at vastly different surface velocities and in opposite directions (e.g. both drums are driven counter-clockwise so that their surfaces in the gap region are moving in opposite directions). As the portion of the photoconductive drum with the electrostatic image on its surface is rotated through the charged toner particle cloud, a visible image is formed on the surface of the photoconductive drum. Where there are multiple colors, there may be multiple photoconductive drums or a single photoconductive drum may be utilized multiple times in sequence, once for each color.
The next step is to transfer the toner image to the print media (paper, transparency, label, envelope, postcard,xe2x80x94whatever substrate gets printed upon and eventually delivered to an output bin), which is typically paper. In many cases, the toner image is transferred directly from the photoconductive drum to the print media by use of mechanical contact and electrostatic forces. However, in the case of a color laser printer, it is sometimes preferable to use an intermediate transfer medium, which may, if used, exist in a drum, belt, sheet, or other form. For the purposes of illustration we shall assume that the intermediate transfer medium is in the shape of a cylinder or xe2x80x9cdrumxe2x80x9d (commonly and hereinafter referred to as an xe2x80x9cITDxe2x80x9d). This approach allows the three or four color images to be built-up on the intermediate transfer drum prior to a final transfer to the paper media. In the case of an intermediate transfer drum, the toner particles are transferred from the photoconductive drum to the intermediate transfer drum by a combination of electrostatic, mechanical, and other forces, and subsequently and similarly transferred to the media. The transfer of toner particles from one surface to another is never 100% efficient, so a toner scraper is positioned to scrape the excess toner from the drum surface after the transfer has occurred. While the toner scraper is commonly used, there are other methods known to those skilled in the art for removing the excess toner from the drum (or other) surface after primary (photoconductor to intermediate transfer medium) or secondary (intermediate transfer medium to the print media) transfer. The cleaned surface can then be reconditioned and used for the next page or the next color of toner.
The transfer of the toner image from the photoconductive drum, or the intermediate transfer drum, to the media is also accomplished by electrostatic, mechanical and/or other forces. For this to occur, the media is brought into close proximity to the rotating drum and moved at a velocity substantially equal to the surface speed of the transferring drum. The electrostatic, mechanical, and/or other forces applied during transfer cause the toner to move from the drum to the media. The toner is only loosely adhered to the media at this point. To bond the toner to the media, heat and pressure are typically applied in a fusing unit to make a permanent bond. It is also known in the art to use pressure alone, or even chemical techniques employing a solvent gas to adhere the toner particles to the media. The fusing unit typically employs two rotating heated drums that are pressed together by springs to apply heat and pressure to the toner and paper as they pass therebetween.
After the fusing operation, the media may be returned to the printing process again, via a duplexing unit, so that both sides can be printed upon. In the alternative, the paper media may be fed to an output tray, either directly or through a collating device that sorts pages to a predetermined order.
At several positions within the printer, paper position sensors track the movement of the paper and serve as reference points for the commencement and termination of various functions in the printer. In addition, other sensors detect consumable levels and movement of other components in the process. Also, various belts, shafts, sheaves, gears, guides and other mechanical devices are used to align and guide the paper media through the printer. Those skilled in the art appreciate that wear, degradation, consumption, faulty components, abuse, environmental factors and many other contributors inevitably lead to degraded performance of such a complicated device.
The effects of such degraded performance are many and varied. By way of example, degraded performance can manifest as blank, streaked, faded, or distorted images and text. In color machines, this can occur on a color by color basis that can result in color distortion. The paper media can be creased, torn or tattered. The entire media may have a dull appearance or may be tinted gray or some other color. The page registration may be misaligned or skewed. When problems of this nature occur, it is often necessary to service the printer by first determining the source of the fault and then repair or replace the faulty components. Repairs may include cleaning, adjusting, or other operations.
The process of identifying the faulty components involves an investigation for which an important piece of evidence is the printed media output from the printer. However, the printed output provides a course bit of after-the-fact circumstantial evidence that may conceal the true source of the fault within a complex printing process.
Beyond looking at the print defects evident in the printed media, another approach that can be used is to open the printer and inspect the various internal components such as the photoconductor, developer sleeve, intermediate transfer medium, transport belts, cleaning mechanisms, fuser, and other internal elements for signs of wear, or damage, and signs of proper or improper operation. For example, if the image on the photoconductor is good prior to primary transfer, one could conclude that the developer is working properly. If the image on the intermediate transfer medium is flawed, one could conclude that the problem happened sometime after development, and before or during primary transfer. This technique can help technicians isolate the source of the problem. Inspecting the print medium as the sheet is processed may be useful but this is not an easy task. As an alternative, one technique is to xe2x80x9cpull the plugxe2x80x9d during the middle of a printing cycle and then open the printer and remove various sheets of media and subassemblies for inspection.
The process of xe2x80x9cpulling the plugxe2x80x9d is more widely known to those skilled in the art of printer design than those skilled in printer service. Knowing exactly when to xe2x80x9cpull the plugxe2x80x9d on a particular printer model under a particular set of circumstances can be a mystery, even to skilled product designers.
The xe2x80x9cpulling the plugxe2x80x9d service approach has worked fairly well in the past. This has been particularly true in the case of printers that operate at a relatively slow output rate. For example, a printer that is capable of printing six pages per minute typically has a per-page print cycle of ten seconds. Perhaps the essential transfer operation occurs in two or three seconds. This is a reasonably long window of time in which a technician can xe2x80x9cpull the plugxe2x80x9d. However, as printers have gained speed and, in particular, where there are multiple colors processed in each cycle, the actual print transfer operation can occur very quickly. This means it is very difficult to xe2x80x9cpull the plugxe2x80x9d at the instant in time most beneficial for the service operation. The problem is further exacerbated in the case where the technician desires to halt the printing process at a particular instant in time that would be most beneficial to the diagnostic procedure.
Thus there is a need in the art for an apparatus and method to interrupt a printing cycle at a predetermined, precise, predictable, and repeatable instant to aid in the diagnosis of product faults and help determine repair requirements.
The need in the art is addressed by the apparatus and methods taught by the present invention. In an illustrative embodiment, a printer is operable to halt a printing operation at a predetermined instant allowing service personnel to open the printer and inspect the printing process for evidence of faulty components and other problems, or for training and educational purposes. The printer includes a printing mechanism and a selector for selecting a predetermined halt time. A controller is coupled to the selector and to the printing mechanism. The controller operates to halt the printing operation at the selected predetermined time.
In a refinement, the predetermined halt time is measured with respect to a particular printer operation event. The event can be selected from a stored list of events. Also, the halt time can be measured as a finite increment of time with respect to the event. Instead of actual time measurements, the predetermined halt time can be measured as a count of finite increments of time about the event. The occurrence of the event can be detected by a sensing device within the printer mechanism. Instead of using time specifically, the predetermined halt time can also be defined as the occurrence of the event per se. The printing operation can be initiated in the conventional way or the printing operation can be commenced automatically upon selection of a predetermined event.